Extinction of marine species

Primary Source: “Species Extinction in the Marine Environment: Tasmania as a Regional Example of Overlooked Losses in Biodiversity” by Graham Edgar, Cath Samson and Neville Barrett.

It’s true that few marine invertebrates and plant species are likely to be listed as threatened in the foreseeable future. It’s not because they are plentiful, as many species are in serious decline. We aren’t finding threatened species because we aren’t really looking that hard… Why worry? Australia isn’t heavily populated and people say that the marine environment is in reasonable condition, apart from a few localised problems like heavy metals and some heavy pest infestation in the main urban rivers. Marine animals usually [but not always] breed well and disperse far and wide. They are pretty hard to kill off. However, things are changing even for the very fertile. Threats to marine species like climate change, invasive species and industrial fishing are accelerating, and there is no place a species can go to get away from them as the effects are often everywhere.

Species declines have often gone unnoticed by different human generations because the changes are so slow compared to the normal human life span. We think the depleted environment we see today was always like that—this is called the “sliding baseline syndrome”. How many marine animals are threatened? Few marine animals are listed as extinct or threatened in Australia, with Tasmania having quite a few of the species that are. This apparently rosy story is an illusion. No-one is really looking to see how bad the problem is. In places like Tasmanian, the majority of local biologists (?50) and biologically trained managers (?30) dealing with marine subjects have to focus mainly on fisheries issues, not on threatened ‘secondary’ species. For example, only one mollusc species Gunns screw shell (Gazameda gunnii) has been listed as threatened. However, MOST of the >1000 Tasmanian mollusc species have not been sighted, or collected alive, during the past two decades. If we take a core sample from the sediments, they show that inshore mollusc biodiversity has decreased from an average of 21 species per 5- cm slice of mud at the start of the twentieth century to 7 species per 5cm slice in 1990. Although NO population data is available for MOST marine species, it is possible to get some idea of general population trends. The data we have indicates major population declines for the majority of species and widespread historical changes.

Catch statistics show that native oysters, commercial scallops, southern rock lobster, orange roughy, eastern gemfish, barracouta, southern bluefin tuna, jack mackerel, school shark, and Bastard trumpeter have declined by >50% over three generations. That’s enough to deserve IUCN “endangered” status—and many have declined by >80% (IUCN “critically endangered” status). Aerial photographs indicate that giant Macrocystis pyrifera has declined significantly along the east coast since 1944. Seagrass beds have declined in area by approximately 25% since the 1950s. All is not lost, the population declines experienced over the past century, may or may not progress to extinction. Local extinction is a lot more likely, than total extinction. Many marine species disperse widely in the larval stage as plankton. Others will spread very little and have very constrained home ranges, like most species of handfish. However, both are vulnerable to events that change the whole ecosystem, like climate change. Climate Change Mean surface sea temperature off the Tasmanian east coast has increased by >1? C since the 1940s, which has been one cause of loss of kelp and urchin barrens. If global warming contributes to another 1–2? C rise over the next century, many species of cool-temperate organisms are likely to disappear altogether. This will include a number of unique Tasmanian species. These species can’t go south to escape, at the bottom of Tasmania there is a deepwater barrier.

Introduced Species

Introduced species are another world-wide problem. Impacts of introduced flora and fauna are largely unknown and unstudied. By taking core samples we can have some idea of the effect it has had on local Tasmanian mollusc (shellfish) species. Introduced species increased from <2% of total shells in 1900 to 50% in 1990. It’s the same story when we look for live molluscs. Surveys across south eastern Tasmania indicated that 39% of total mollusc numbers and 83% of total mollusc biomass belonged to introduced species. That means that when you look at the bottom the vast majority of the shells you see are alien, not local. That’s pretty ‘uncool’ [Amy tells me I’m not allowed to do that word even if my generation invented it, I’m too old and that’s uncool], because some of those species like the live-bearing seastar and handfish, really aren’t found anywhere else. We are losing not just Tassie species, but whole kinds of animals that are part of the world’s natural heritage. Fishing Targeted fish catches will rarely cause extinction of a species because the cost of fishing eventually exceeds the economic returns. This is called “commercial extinction”. Indirect effects of fishing, however – habitat damage, bycatch, and trophic cascades – are not closely linked to economics and can potentially lead to extinction. Oyster beds were lost as a habitat type statewide around 1890 following extensive dredging. Inshore scallop beds were depleted during the twentieth century. We don’t really know what damage this caused to populations of unique Tasmanian species that relied on these habitats. Live-Bearing seastars – driven to the brink by NZ cushion stars? Page 16 In 1989, large schools of orange roughy were discovered aggregating on approximately 70 seamounts off southern Tasmania. Trawling commenced off the deepwater seamounts that rise 300–600m from the continental slope in water depths of 1000–2000 m. During the initial “gold rush” period, all shallow (<1000 m depth) seamounts were heavily trawled, some more than 3000 times. A virtually unregulated trawl fishery blossomed for only 5-years before the fishery collapsed. The coral gardens removed by this process were likely to be unique to the area and have been heavily damaged if not destroyed. It is likely that rare species relying on that habitat have also been badly affected or destroyed. Unfortunately, it’s a bit too deep to go down and check it out easily. Losses to bycatch can also be important. Shark species appear particularly vulnerable to bycatch threats because of slow growth, late onset of sexual maturity, direct reproduction, low fertility, and low natural mortality. Indirect effects of fishing remain virtually unknown in Tasmania. In Tasmania annual surveys of plants and animals in four MPAs have revealed direct effects such as increases in rock lobsters and trumpeter, and indirect effects such as declines in population numbers of abalone. Potentially damaging sea urchins have decreased by 70% in the largest MPA over 12 years. This has not yet translated to detectable changes in algal cover. In New Zealand’s Leigh marine reserve, the only temperate MPA worldwide that has been studied for more than 20 years, increasing rock lobster and fish predation on grazers ultimately transformed sea urchin barren habitats to macroalgal forests after 15 years. Conclusion Our few marine scientists are mostly busy studying more economically important fished species. We don’t really check on a broad range of marine animal populations. As a result, you can’t assume that marine animals are immune from extinction risk. We always have to be careful and watchful.